Recent studies have shown that there exists a family of mammalian protein kinases structurally and functionally related to the yeast cell cycle regulatory kinase cdc-2. At least five members of the cdc2 family, cdk1 through cdk5, have been identified in mammalian cells. Our laboratory is focused on studying the properties of cdk5 which, in contrast to either cdk 1 or cdk2, cdk 5 is expressed at high levels both in terminally differentiated neurons and many cultured cell lines. We have generated an antipeptide antiserum specific for cdk 5, which can specifically immunoprecipitate cdk 5 kinase from mammalian cell extracts, and binds specifically to cdk 5 in Western blotting studies. Using this antiserum, we have established that cultured human tumor cells express enzymatically active cdk 5 kinase. We are currently attempting to reconstitute active cdk5 enzyme in vitro after expression in baculovirus and subsequent purification, which will allow identification and expression of key regulatory molecules that modulate activity. In addition, we have cloned and characterized cDNA and genomic clones encoding the Drosophila homolog for mammalian cdk5 and mapped the gene to a band on chromosome 2, thereby providing the information needed to generate flies genetically deficient in the cdk5 gene.